Long exposures: water, clouds and light trails

A long exposure records movement over time in a single frame. Rushing water becomes silky, waves turn to mist, clouds streak across the sky and headlights trace lines through the dark. These effects are among the most recognizable in landscape photography, and also among the most overused. This lesson explains how to choose shutter speeds for different effects, estimates how far things move during an exposure, covers technique for stable long exposures in the field, and discusses when a long exposure helps a picture and when it becomes a cliché. Lesson 5 covered the ND filters that make long exposures possible in daylight.

Shutter speed as a creative choice

In most photography, shutter speed is chosen to avoid blur. In long-exposure landscape work, it is chosen to create a specific amount of blur. The key question is how far a moving element travels during the exposure, relative to its size and the frame. A wave that moves 1 m during a 1/4 s exposure keeps much of its texture; one that moves 1 m during a 30 s exposure, back and forth many times, disappears into a smooth mist.

The distance moved is simply speed multiplied by time:

On the sensor, the blur length depends on magnification, as discussed in earlier courses: for a subject at distance \( d \) photographed with focal length \( f \), the blur on the sensor is about \( v t f / d \). For long exposures, though, it is often more useful to think about the blur relative to the size of the subject or the frame.

Water: from texture to silk

Different shutter speeds give different looks to moving water:

For waterfalls, the water accelerates as it falls. Ignoring air resistance, water falling a height \( h \) reaches a speed of about:

where \( g ≈ 9.8 \) m/s². This helps estimate how much the water will blur.

Because bright white water can easily overexpose, expose to keep highlight detail in the falls, checking the histogram and highlight warnings, and lift shadows later.

Waves and coastal long exposures

At the coast, exposures of about 0.3 to 1 s capture the motion of retreating waves as streaks around rocks, which many photographers find more interesting than total smoothness. Timing matters: start the exposure just as a wave begins to retreat. Longer exposures of 30 s or more turn the sea into a smooth plane, which can isolate rocks, piers and sea stacks in a minimalist composition.

Coastal long exposures demand safety awareness. Watch the sea for several minutes before setting up, never turn your back on waves on exposed shores, and know the tide (Lesson 2 covered tides; Lesson 11 covers safety). Rogue or sneaker waves, much larger than average, have swept photographers from rocks on both coasts of North America. Salt spray also coats lenses and filters quickly; check and clean between frames.

Clouds

Moving clouds streak across the frame during exposures of tens of seconds to several minutes, giving a sense of motion and time. The effect depends on cloud speed, direction relative to the camera and focal length. Clouds moving toward or away from the camera create radiating lines; clouds moving across the frame create horizontal streaks.

Cloud streaks need a bright enough subject and a dense ND filter in daylight, and they work best with defined clouds. A clear blue sky or uniform gray overcast shows little motion.

Light trails

After dark, moving lights draw lines. Car headlights and taillights on roads, boats on lakes and rivers and even flashlights carried by hikers on a trail create trails that add energy and show human presence in a landscape. Exposures of 10 to 30 s at night usually capture complete trails from passing vehicles. The trail's length is the vehicle's speed times the exposure time, limited by how long the vehicle is in the frame.

Twilight, during the blue hour, is often the best time for light trails in landscapes: the sky still has color and detail, while artificial lights are bright enough to stand out. Stand only where it is safe and legal, well off the road, and never photograph from road shoulders where drivers cannot see you.

Technique for sharp long exposures

Long exposures blur moving elements on purpose, but the stationary parts of the scene must stay sharp. Common causes of unwanted blur are tripod movement, wind, vibration from the camera itself and touching the camera. Good practice includes:

• A solid tripod, with the center column down and the heaviest leg sections extended first. Hang a bag from the hook to add weight, but not so it swings.
• A remote release or self-timer, so pressing the shutter does not shake the camera. For exposures over 30 s, use bulb mode with a lockable remote or the camera's built-in bulb timer.
• Mirror lock-up or an electronic first-curtain shutter to avoid vibration from the shutter mechanism.
• Shielding from wind with your body, a bag or a nearby rock. On boardwalks and bridges, footsteps and traffic shake the tripod.
• Turning off image stabilization on a tripod, unless the lens manual says it detects tripod use, because stabilization can introduce drift.
• Focusing and composing before fitting a dense ND filter, then switching to manual focus so it does not change.

Noise, heat and battery life

Long exposures warm the sensor and increase noise, especially hot pixels: bright single pixels that appear in long exposures. The camera's long-exposure noise reduction takes a second exposure of equal length with the shutter closed, a dark frame, and subtracts it, which removes hot pixels but doubles the time. On a cold night this is less of a problem, while in summer heat it can matter more. Long exposures also drain batteries, particularly in cold weather, so carry spares in an inside pocket (Lesson 11).

Calculating exposure in bulb mode

With dense ND filters, the required exposure often exceeds 30 s. A simple method is to meter without the filter, then apply the filter factor from Lesson 5. When the light is changing quickly, such as at sunset, recheck often. In very low light, take a short high-ISO test frame to measure exposure quickly, then convert back to low ISO: each doubling of ISO halves the needed time.

When a long exposure helps, and when it doesn't

Silky water and smooth seas became so popular that they risk looking generic. A long exposure should support the picture's idea. It works well when it simplifies a busy scene, reveals patterns of movement that the eye cannot see, conveys calm or the passage of time, or separates a static subject from a moving background. It works badly when it removes interesting texture, such as the power of a storm wave, or makes every waterfall look the same. Try several shutter speeds and compare: sometimes 1/8 s with visible structure is stronger than 30 s of mist.

Long exposures also raise honesty questions in documentary or scientific contexts. Viewers generally understand that smooth water is a photographic effect, but a long exposure that hides crowds at a busy site can misrepresent how a place is experienced. Captions can clarify.

Planning a long-exposure outing

A simple checklist helps long-exposure outings go smoothly: a sturdy tripod and a weight or bag to hang from it; ND filters in several strengths, cleaned and in a case; a remote release with a lockable button or timer; spare batteries; lens cloths for spray and rain; a headlamp for twilight work; and a note of tide times and safe access on coasts. Arrive early to choose positions while you can see the ground clearly, and test exposure times before the light peaks so you are ready when it does.